Tubes and heat-shrinkable tubes have conventionally been used extensively in applications such as the insulation or marring protection of terminals and junctions of electric wires and cables and the protection of joints of steel pipes and other various pipes against water, corrosion, etc.
Of such applications, those which require waterproofing and corrosion protection employ heat-shrinkable tubes having a pressure-sensitive adhesive layer or an adhesive layer, preferably an adhesive layer, formed on the inner surface thereof.
In some applications, there are cases where the inside of a tube or heat-shrinkable tube having been used for covering is required to be examined visually. For such applications, crosslinked tubes or heat-shrinkable tubes composed of transparent resins, such as a polyvinyl chloride resin, a polyethylene terephthalate resin, and an ionomer resin, are used.
However, the tubes and the heat-shrinkable tubes composed of a polyvinyl chloride resin occasionally have a problem of plasticizer migration. The heat-shrinkable tubes composed of a polyethylene terephthalate resin have a problem that during heat shrinkage operations, they not only shrink in the radial direction but shrink considerably in the lengthwise direction. Further, since the heat-shrinkable tubes composed of a polyethylene terephthalate resin have insufficient flexibility, they have problems, for example, that the handling thereof is difficult when they have large wall thicknesses.
Known ionomer resins for a crosslinked tube and heat-shrinkable tube have a structure in which a copolymer of ethylene and acrylic or methacrylic acid has been intermolecularly crosslinked with metal ions such as zinc ions or sodium ions.
For producing a crosslinked tube made of an ionomer resin, the ionomer resin is shaped into a tube using a melt extruder or the like and the tube is then crosslinked by irradiation with an ionizing radiation such as electron beams.
For producing a heat-shrinkable tube made of an ionomer resin, the ionomer resin is shaped into a tube using a melt extruder or the like, the tube is then crosslinked by irradiation with an ionizing radiation such as electron beams, and the diameter of the crosslinked tube is expanded under high-temperature conditions by, for example, introducing compressed air into the tube and is then fixed by cooling.
A heat-shrinkable tube made of an ionomer resin and having an adhesive layer on the inner surface thereof can be obtained in the same manner as in the production of the heat-shrinkable tube made of an ionomer resin alone, except that an adhesive resin, such as a polyamide, and the ionomer resin are shaped by melt co-extrusion.
The heat-shrinkable tube composed of a resin molding comprising an ionomer resin is free from the plasticizer migration problem accompanying the heat-shrinkable tubes of a polyvinyl chloride resin and from the problems of lengthwise shrinkage and handling difficulty due to insufficient flexibility accompanying the heat-shrinkable tubes of a polyethylene terephthalate resin.
However, the crosslinked tube or heat-shrinkable tube made of an ionomer resin and the heat-shrinkable tube made of an ionomer resin and having an adhesive layer on the inner surface thereof have had a problem that, in the production thereof, Lichtenberg discharge marks generate within the ionomer resin layer and/or at the interface between the heat-shrinkable ionomer resin tube layer and the adhesive layer during the step of electron beam irradiation, thereby to considerably impair the clarity and appearance of the product.